Rotationally Resolved Diode Laser Jet Spectroscopy of Propadienone (CH 2 CCO) in the 3 Band Region P. J. O’Sullivan, R. J. Livingstone, Z. Lui, P. B. Davies Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK

Introduction Propadienone is a member of the cumolenone series (H 2 C n O), is short lived and has a non rigid kinked backbone. Microwave spectra measured in the low frequency ν 12 bending mode : Brown, Godfrey and Champion, J. Mol. Spec., 123, 93 (1987) Infrared spectra in N 2 and Ar matrices; no rotationally resolved gas phase spectra Pulse Pyrolysis Infrared Laser Jet Spectroscopy used to produce a rotationally resolved spectrum

The Kinked Structure of Propadienone (PPD)

Double-Well Potential for PPD 1 Vibrational levels split by quantum tunnelling through inversion barrier. Lowest pair of vibrational levels separated by 3.7GHz (0.123cm -1 ). Splitting of subsequent levels dependent on energy in relation to the inversion barrier and component of vibration along inversion axis. 1 R.D. Brown, P.D. Godfrey, R. Champion J.Mol.Spec 123, 93 (1987)

Wavenumber (cm -1 ) Absorbance Infrared Spectrum of Matrix-isolated PPD 1 1 O.L. Chapman, M.D. Miller, S.M. Pitzenberger J. Am. Chem. Soc. 109, 6867 (1987) argon matrix Freq. (cm -1 ) Intensity nitrogen matrix Freq. (cm -1 ) Intensity

Experimental

Experimental – Flash Pyrolysis Nozzle 1 Ceramic tube: 1mm i.d., 2mm o.d., 42mm long Heating coil: thin NiCr wire, insulated by MgO, inside a stainless steel sheath. 10mm long, 3.5 Ω Applied power of 10W → 1500K inside tube. 1. P. Chen, S.D. Colson, W.A.Chupka, J.A. Berson J.Phys.Chem. 90, 2319 (1986)

Experimental – Signal Processing A: Averaged absorption signal at a peak position. B: Trace A with a 1.4ms time delay. C: The subtraction of Trace B from Trace A. D: Resultant smoothed Trace. Gas Pulse A B C (=A-B) D 35 Hz msec

Synthesis of Propadienone based on pyrolysis of either (ii) diazotetronic acid: Synthesis of Propadienone based on pyrolysis of either (i) 2,2-dimethyl-5-methylene-1,3-dioxan-4,6-dione: Chemistry Synthesis of Propadienone based on pyrolysis of either (iii) acrylic anhydride:

Chemistry All three starting compounds have to be synthesised in the laboratory but only acrylic anhydride (iii) is stable for more than a few days and was therefore the preferred reagent. Conditions:  Acrylic anhydride heated to 80 o C  Nozzle temperature of 600 o C (heating length = 20mm)  Backing pressure of argon = 2.5 bar  Nozzle-laser distance = 5mm

Spectral Details Continuous spectral coverage from cm -1 to cm -1, using several modes from a single diode laser. Over 180 intense lines observed and measured with an estimated accuracy of cm -1. Several series of regularly spaced lines separated by the order of (B + C) = cm -1 for the ground state. Absorption signals disappeared below 450 o C.

Sample Spectra Frequency (cm -1 )

Loomis-Wood Plot of Hypothetical PPD R-branch Q-branch P-branch Upper state constants identical to ground state constants: Very regular series of lines with no distortion, as expected. Indication of approximate magnitude of asymmetry doubling. Q-branch expected, although not intense and may be dispersed in real spectrum.

Loomis-Wood Plot PPD cm -1 (B+C = 0.29cm -1 )

Q-branch structures in CCO Antisym. Stretch of Ketene 1 1. J.L. Duncan, A.M.Ferguson, J.Harper, K.H.Tonge, F.Hegelund J. Mol. Spec. 122, 72 (1987) Q-branch structures of isotopically-substituted ketenes suggest an unusual arrangement of the K a sub-bands. If propadienone is similarly erratic, this could account for:  A dispersed and unidentified Q-branch  The apparent lack of a relationship between the observed series in the Loomis-Wood plot.

Ground State Combination Differences (cm -1 ) for PPD J Kc’ – J Kc” Microwave Infrared Obs-Calc (10 4 ) K a = – – – – – – – – – – – – – – J Kc’ – J Kc” Microwave Infrared Obs-Calc (10 4 ) K a = – – – – – – – – – – – – – – – – – – – –

Molecular Constants (cm -1 ) of PPD υ 12 = 0 υ 12 = 1 Ground State υ 3 =1 Ground State υ 3 =1 v (32) (32) A (2) (113) (3) (117) B (1) (6) (1) (6) C (1) (5) (1) (5) Δ (amuǺ 2 ) D J (x10-8)5.82(10)-60.9(13)5.79(10)-60.5(13) D JK (x10-5)-2.621(13) (17)-2.596(10) (17) D K (x10-1) (33)4.8595(16) (33)4.8486(16) D 1 (x10-9)7.61(17)-4642(21)-7.17(17)-4669(21) H kj (x10-7)1.03(67)-981.5(24)-9.21(67)-977.5(25) κ Lines Rms

Rotationally resolved spectrum of the ν 3 C-O stretching fundamental of propadienone recorded Combination Differences and Loomis Wood plots lead to assignment of many K a = 0 and 1 P-and R-branch lines Structure of the IR spectrum and of the upper state constants indicate a significant change in the ν 12 inter-conversion potential for the upper level and/or perturbations of the upper level Next step? Analysis of congested region about 4.5 cm -1 below band origin – possible intersystem v 12 = 0-1 lines ( Δ K a = -1) Conclusions

Ray Livingstone Zhuan Liu Paul O’Sullivan Iain Bell £££££ EPSRC Acknowledgements